igb: Add support for byte queue limits.

This adds support for byte queue limits (BQL)

Since this driver collects bytes count in 'bytecount' field, use it also
in igb_tx_map()

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>